The basic idea is to graft multiple tree's together, forming the shapes you want, and then let them grow while pruning and caring for them, possibly over many years. Sheesh. I can't even imagine this kind of patience. I spent an hour just clicking on links from google images.
Right now in Austrailia Peter Cook and Becky Northey are continuing this patient artform as the Pooktre tree shapers They have some incredibly cool stuff. I can just imagine stumbling upon one of their tree people in the dark, and swearing for the rest of my life that I met a Dryad.